Various processes exist for the production of reduced iron or alloy iron, and processes of reduction in a solid state using carbon as the reducing agent are widely employed throughout the world because of the low operation and equipment costs and the ease of actual operation. Examples of such processes are those wherein reduction is accomplished by heating while rotating a compact of iron oxide and carbon powder on a refractory material with a rotary kiln or the like, and those wherein reduction is accomplished by putting on a moving hearth and heating by high temperature gas from above, as in, for example, the rotary hearth process.
Among such processes, the rotary hearth process is the one which offers the highest productivity. The rotary hearth process involves a system composed mainly of a firing furnace of a type in which a disk-shaped refractory hearth lacking the center portion is rotated at a fixed speed on rails, under an anchored refractory ceiling and walls (hereinafter referred to as “rotary hearth furnace”), and it is used for reduction of metal oxides (hereinafter referred to as “rotary hearth-type reducing furnace”). A rotary hearth furnace has a rotating disk-shaped hearth with the center portion lacking. The diameter of the disk-shaped hearth is from 10 to 50 meters and its width is from 2 to 6 meters.
The basic outline of operation in a rotary hearth process is as follows. First, the metal oxides such as ore or dust or sludge as the starting materials are mixed with an amount of carbon-based reducing agent necessary to reduce the oxides, and then compacts are produced using a granulating machine.
The starting materials used are metal oxides such as ore powder or metal oxide dust, and carbon as a reducing agent. In the production of reduced iron, fine iron ore such as pellet feed ore is used. Carbon is used as the reducing agent, but it preferably has a high non-volatile carbon (fixed carbon) content at up to about 1100° C. as the temperature at which a reduction reaction occurs. Coke powder or anthracite coal is suitable as such a carbon source.
Iron oxide-containing powder as the starting material is mixed with carbon-containing powder. The mixture is then compacted and supplied to a rotary hearth furnace spread on the hearth. In the rotary hearth furnace, the hearth is rotated and the compacts spread on the hearth are rapidly heated at a temperature as high as 1300° C. for 5 to 20 minutes as it is moved through each of the high-temperature sections of the furnace. The reducing agent (carbon) mixed in the compact causes reduction of the metal oxide in the compact during this time, producing the metal. The metallization rate differs depending on the metal to be reduced, but for iron, nickel or manganese it is over 70%, and it is above 30% even for reduction-resistant chromium. When treating dust generated by an iron-making process, the reduction reaction is combined with volatilizing removal of impurities such as zinc, lead, alkali metals and chlorine, thus facilitating recycling to a blast furnace or an electric furnace.
Because the compacts are stationary on the hearth in the rotary hearth furnace, an advantage is provided in that the compact does not readily disintegrate in the furnace. As a result, the problem of the powdered starting material adhering to the refractory material does not occur, and an advantageous bulk product yield is achieved. High productivity and the ability to use inexpensive coal-based reducing agents or powder starting materials are additional reasons for the increasing employment of this process in recent years.
The rotary hearth process is effective for reduction and impurity removal treatment of dusts generated in a blast furnace, converter or electric furnace or thickener sludge from a rolling process, and it may also be used as a dust-treatment process or as an effective process for metal resource recycling.
The equipment comprises a starting material pre-pulverizer, a starting material mixer, a granulator, a rotary hearth-type reducing furnace, a off-gas treatment apparatus and a reduced compact cooling apparatus.
As mentioned above, a reduction process wherein the compacts are put on a moving hearth and heated from above by the high-temperature gas, as in a metal oxide reduction process such as a rotary hearth process, involves no movement of the compacts on the hearth, and therefore destruction or powdering of the compact is minimized, such that the process is excellent from the standpoint of producing a strong reduced iron compacts (granules) and from the standpoint of productivity or production cost, and hence economical production can be achieved. However, further improvement in productivity and quality is desired. Specifically, it is necessary to accomplish more efficient reduction to increase productivity, while satisfying the physical conditions which facilitate use of the obtained reduced iron compact in later steps.
As a reduced compact (hereinafter referred to either as reduced iron compact or reduced iron pellets) is not used directly as a product, it must undergo final reduction and melting in later steps. In particular, with reduced iron pellets produced by a rotary hearth process, sulfur is absorbed from the carbon source into the metallic iron, resulting in a sulfur content of 0.1-0.3% in the reduced iron, such that it is unsuitable for direct use as a steel product. A desulfurization function is therefore necessary in the final reduction and melting step. An iron-making blast furnace has a desulfurizing function with the reduction and melting, and therefore production of molten iron using the reduced iron pellets in admixture with other starting materials in the iron-making blast furnace is an economical method for iron production.
For use in a blast furnace, however, it is necessary to produce reduced iron pellets with high strength. The reason for this is as follows. A large amount, as much as 2000-8000 tons, of ore and coke may build up in a blast furnace. A significant load therefore acts on the reduced iron pellets in the blast furnace, and the required crushing strength can be as high as 5×106 to 6×106 N/m2 or greater.
Methods of producing high-strength reduced iron pellets by the rotary hearth process already exist in the prior art, as disclosed in Japanese Unexamined Patent Publication No. 2000-34526 and Japanese Unexamined Patent Publication No. 2000-54034, previously filed by the present inventors. The operation according to the technology disclosed in these publications is effective for production of high-strength reduced iron pellets, and it is therefore an indispensable technique for production of reduced iron pellets for use in blast furnaces. The reduced iron pellets have exceedingly high crushing strength and can therefore be directly used in the blast furnace.
However, the problem with operation based on these disclosed techniques has been a lack of fine management of the starting material conditions and reaction conditions. That is, even with these techniques, insufficient management of the reaction time has often resulted in reduced iron pellets with inadequate strength. Furthermore, the reaction time management is not quantitative, leading to a prolonged reaction time and therefore overconsumption of energy for heating and reduction. Another problem has been insufficient management of the conditions of the starting material components, or the conditions such as the size of the compact supplied to the reduction furnace for the rotary hearth process. A new technique which overcomes these problems has therefore been desired.
It has been the experience of the present inventors that when the iron oxide starting material is not carefully selected, the reduced iron compact product undergoes severe powderization even with appropriate operating conditions in the rotary hearth process. The present inventors therefore conducted numerous experiments while varying the starting material formulating conditions. As a result, it was found that of the iron oxide starting materials used, those with the highest ferric oxide (Fe2O3) blending ratios gave products (reduced iron compacts) with the highest powder ratios.
Here, “product” refers to the compact which is reduced after heating reduction (reduced iron compact), and it includes bulk reduced products, i.e. bulk reduced iron compacts or reduced iron pellets, as well as powdered reduced products, i.e. powdered reduced iron compacts (hereinafter referred to as “powder”). The powder ratio is the ratio of the mass of reduced product which passes through a 2 mm sieve with respect to the total mass of the reduced product before passing through the sieve.
For example, experiments conducted by the present inventors demonstrated that severe generation of powder occurs when the proportion of ferric oxide in the starting material powder exceeds 60%, for pellets produced from raw material powder with a mean particle size of 45 μm using a pan-type granulator. Moreover, with a ferric oxide proportion of greater than 70%, the powder ratio of the product (reduced iron compact) was as high as 15-25% even if the operating conditions of the rotary hearth-type reducing furnace were satisfactory. Further investigation by the present inventors confirmed that the powder generated in the furnace is inferior in terms of reduction rate and dezincification. This was because the powder has a large specific surface area and more easily contacts the combustion gas in the furnace on the hearth, thereby being affected by the oxidizing atmosphere of carbon dioxide gas and water vapor in the combustion gas, and being inhibited the reduction reaction. In other words, powderization of the compact creates the problem of a lower proportion of highly valuable bulk product (bulk reduced compact) and a lower average reduction rate of the product. As a result, while it has been known that inhibiting such powderization is important in order to reduce compacts of containing ferric oxide to obtain products having a high reduction rate with a metal ratio of 75% or greater, no effective countermeasure has existed in the prior art.
As no effective means for solving these problems has existed in the prior art, no efficient reduction treatment has been carried out to prevent powderization. Consequently, a new technique for reducing powderization of compacts has been desired in reduction of ferric oxide-containing iron oxide compacts in rotary hearth-type reducing furnaces.
It is therefore an object of the present invention to 1) efficiently obtain reduced iron compacts with high crushing strength and 2) to efficiently reduce iron oxide starting materials containing ferric oxide in order to obtain reduced iron compacts with low powder and high reduction rates, in a solid reduction-type heating reducing furnace such as a rotary hearth-type reducing furnace, as well as to achieve reduction melting of reduced iron compacts in blast furnaces.